1. Field of the Invention
The present invention relates generally to the selective modification and control of a patient's body temperature. More particularly, the present invention provides methods and apparatus for treating hypothermia or hyperthermia by inserting a catheter into a blood vessel of the patient and selectively transferring heat to or from blood flowing through the vessel.
2. Description of the Background Art
Under ordinary circumstances the thermoregulatory system of the human body maintains a near constant temperature of about 37.degree. C. (98.6.degree. F.). Heat lost to the environment is precisely balanced by heat produced within the body.
Hypothermia is a condition of abnormally low body temperature. Hypothermia can be clinically defined as a core body temperature of 35.degree. C. or less. Hypothermia is sometimes characterized further according to its severity. A body core temperature in the range from 32.degree. C. to 35.degree. C. is described as "mild" hypothermia, 30.degree. C. to 32.degree. C. is called "moderate," 24.degree. C. to 30.degree. C. is described as "severe," and a body temperature less than 24.degree. C. constitutes "profound" hypothermia. Although the above ranges provide a useful basis for discussion, they are not absolutes and definitions vary widely in the medical literature.
Accidental hypothermia results when heat loss to the environment exceeds the body's ability to produce heat internally. In many cases, thermoregulation and heat production are normal but the patient becomes hypothermic due to overwhelming environmental cold stress. This is a relatively common condition, often resulting from exposure to the elements. Hypothermia may also occur in patients exposed to mild cold stress whose thermoregulatory ability has been lessened due to injury or illness. For example, this type of hypothermia sometimes occurs in patients suffering from trauma or as a complication in patients undergoing surgery.
Hypothermia of either type is a dangerous condition which can have serious medical consequences. In particular, hypothermia interferes with the ability of the heart to pump blood. Hypothermia may be fatal for this reason alone. Additionally, low body temperature seriously interferes with the enzymatic reactions necessary for blood clotting. This sometimes results in bleeding that is very difficult to control, even when normal clotting factor levels are present. These effects and other adverse consequences of hypothermia lead to drastically increased mortality rates both among victims of trauma and in patients undergoing surgery.
Simple methods for treating hypothermia have been known since very early times. Such methods include wrapping the patient in blankets, administering warm fluids by mouth, and immersing the patient in a warm water bath. While these methods are very effective for mild hypothermia, more intrusive methods have been developed for treating severe and profound cases of hypothermia. In particular, methods have been devised to effect direct heating of a patient's blood. Most commonly, blood is withdrawn from patient's circulation, passed through external warming equipment, and reinfused back into the patient. Alternatively, the use of heated catheters has been proposed, where a catheter having a heating element near its distal end is inserted into the patient's vasculature and heat directly transferred into the patient's circulating blood.
While the direct heating of patient blood can be highly effective, even in treating severe and profound cases of hypothermia, it has been observed by the inventor herein that the excess transfer of heat can cause the patient's temperature to rise above normal body temperature, resulting in hyperthermia. Hyperthermia can occur, for example, when a hypothermic patient's metabolism begins to produce substantial amounts of heat at the same time heat is being transferred directly to the blood.
It would therefore be desirable to provide methods for treating hypothermia which further provide for treatment of accidental or incidental hyperthermia. In particular, it would be desirable to develop systems and methods for transferring heat to the blood where heat can be optionally removed if the patient blood or body temperature exceeds a target level. Such methods and devices will preferably employ a catheter for direct heat transfer into circulating blood, but could also be useful with methods where blood is heated externally from the patient's body. Such systems and methods should further be useful for the treatment of patients who are initially hyperthermic, where the methods and systems provide for initial cooling of the blood and optional heating of the blood should the patient blood or body temperature fall below a target temperature.